$\dfrac{7}{8} - \dfrac{1}{4} = {?}$
$ = {\dfrac{7 \times 1}{8 \times 1}} - {\dfrac{1 \times 2}{4 \times 2}} $ $ = {\dfrac{7}{8}} - {\dfrac{2}{8}} $ $ = \dfrac{{7} - {2}}{8} $ $ = \dfrac{5}{8}$